1. Field of the Invention
The invention relates to a gravity base structure for an offshore platform in arctic regions, the structure comprising a monolithic concrete caisson closed by a top slab and by a bottom slab resting on the sea bed, the caisson exhibiting at its circumference a configuration of vertical teeth capable of withstanding icebergs colliding with it and absorbing the impact energy.
2. The Prior Art
The exploitation of hydrocarbon fields in arctic seas, or in iceberg infected zones, entails many problems due to the danger of collision with extremely large icebergs.
Platforms are known which are capable of withstanding pack-ice by breaking the latter up on inclined parts provided over a height corresponding to the thickness of the ice layer, as described in Patent FR-A-No. 2,562,112. However, such platforms are not well adapted to withstand the impacts of icebergs, the submerged part of which is extremely large and may hit the lower portion of the platform, causing the latter to slip on the sea bed, if not more serious destruction.
In order to prevent such accidents, defenses are provided at a distance from the platform with the purpose of modifying the course of the icebergs and moving them away from the protected region. However, this solution is extremely expensive and requires constant surveillance of the region in order, if necessary, to destroy the largest icebergs before they come into contact with the defenses.
U.S. Pat. No. 4,422,804 found a relatively simple solution for platforms which have to withstand the impact of large icebergs. The platform consists of a monolithic massive concrete structure exhibiting an array of cylindrical or prismatic compartments extending vertically from a foundation slab resting on the seabed and closed by a cover slab. This structure generally extends above the surface of the water and supports the deck by means of columns. The circumference of the structure exhibits pointed or rounded teeth extending vertically, which form a reinforced part of the external compartments thus forming the structure capable of withstanding the impacts of icebergs and possibly able to crush them.
The compartments have thick walls and occupy the totality of the structure. The walls stiffen the whole of the structure and participate in withstanding the forces. The great thicknesses of concrete are also subjected to substantial stresses due to the temperature of the oil stored in the compartments and require appropriate prestressed reinforcements.